1. Field of the Invention
The present invention relates to a regulator incorporating an overvoltage protection circuit, and more particularly to a technique of reducing the electric power consumption thereof.
2. Description of Related Art
Conventionally, stabilized direct-current power supplies incorporate a regulator as means for yielding a constant output voltage without being influenced by the input voltage, the load current, the ambient temperature, etc. In particular, chopper-type regulators (switching regulators) that produce a constant output voltage by varying the duty of an output drive circuit offer the advantages of very low heat loss and efficient operation in cases where there is a great input-output voltage difference. Such regulators generally incorporate an overvoltage protection circuit (see, for example, Japanese Patent Application Laid-open No. 11-289759).
Certainly, with a conventional regulator incorporating an overvoltage protection circuit, when a voltage higher than the threshold value appears in the output stage, as when the load become open-circuited, the output operation of the regulator can be stopped. This helps prevent breakdown etc. of circuit elements due to overvoltage.
However, since the conventional regulator is so configured as to achieve overvoltage protection by constantly monitoring the output voltage without regard to whether it is in an operation-enabled or -disabled state or the state of the input and output voltages, it has the disadvantages of consuming unnecessary electric power in the overvoltage protection circuit. For example, in an overvoltage protection circuit that detects overvoltage based on the result of comparison between a division voltage obtained through resistor division of the output voltage and a predetermined reference voltage, electric power is lost by being steadily consumed by resistors, resulting in unnecessarily high electric power consumption. Thus, when the conventional regulator is adopted as a stabilized direct-current supply in a battery-operated electronic apparatus (such as a cellular phone), inconveniently, it tends to waste the electric power stored in the battery, unduly shortening the duration the electronic apparatus can be used.